In regards to new features, since there are so many, the best thing for you to do is just read through the release notes for every release since you last used C2.
The counter at the bottom of the website shows over 600,000 copies of C2 have been downloaded.
The average C2 user loves C2, as far as I know.
C2 has great commercial potential. Look into Mortar Melon, Super Ubie land, and Angvik as some examples I can think of off the top of my head.
I'd say C2's killer features (compared to other tools) are the brilliant layout editor, the event sheet system, its behaviors system, and the wide range of exports.
I would consider C2 to be a very successful project - Scirra live off it full-time.
I believe HTML5 is looking to be the future of game development.
Not sure about your last question.
Hope that helps!